Telephone-exchange system.



No. 646,694. Patented Apr. 3, I900.

W. D. GHAliKY &. E. E. CLEMENT.

TELEPHONE EXCHANGE SYSTEM.

, (Appligation filed Aug. 20, 1898.)

2 Sheets-Sheet (No Model.)

No. 646,694. Patented Apr. 3, I900. W. D. GHARKY &. E. E. 'CLEMENT.

TELEPHONE EXCHANGE SYSTEM.

(Applic'ation filed Aug. 20, 1898.) (No Model.) 2 Sheets-Sheet 2.

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NITE STATES PATENT CEEICE.

WILLIAM D. GHARKY, OE PHILADELPHIA, PENNSYLVANIA, AND EDWARD E. CLEMENT, OF WASHINGTON, DISTRICT OF COLUMBIA, ASSIGNORS To THE sUN'ELECTEIC MANUFACTURING COMPANY, OF NEW JERSEY.

TELEPHONE-EXCHANGE SYSTEM.

SPECIFICATION forming part of Letters Patent No. 646,694, dated April 3, 1900.

Application filed August 20,1898. Serial No. 689,119. (No made.)

To aZZ whom fi ming/concern.- first call for connection, Which is sent by the Be it known that we,WILLIAM D. GHARKY, act of taking down the subscribers receiving= residing in the city and county of Philadeltelephone for use, to the clearing-out signals,

phia, State of Pennsylvania, and EDWARD E. which are given by the replacement of the re- CLEMENT, residing in Washington, in the Disceiving-telephone upon its hook. The opertrict of Columbia,citizens of theUnited States, ators are divided into two sets, the duty of have invented a new and usefulImprovement one set being to answer calls, while that of in Telephone-Exchange Systems, of which the the other is to complete connections initiated following isa specification. by and according to instructions received Io Our invention relates to improvements in from the first set. The work is thus simpii- 6o telephone-exchange systems in which subfied and expedited. The answering of a call, scribers lines radiate from one or'more comthe passing of the number wanted, and the mon central offices,each.line being provided sending out of a ringing-current over the at the subscribers end with means for receivcalled line are all done in any particular in- [5 ing or transmitting signals and a telephone stance by one and the same operator. Re 6 set and at the central-office end with signalsponsibility for omissions or delays is thus receiving means and connective means, and fixed. The operators are provided with sueach central oflice being provided with conpervisory signals always in service, which ennecting means, with telephone sets, and with able them to see at a glance the condition of 20 signal sending and receiving means, all capaa line at any time, Whether the subscriber 7o bio of use with any of the lines. I has called, whether he has been connected,

The direct objects of the invention are the whether he has answered or not,'and whether elimination of unnecessary apparatus, the one subscriber or both afterconversin-g have simplification of all apparatus retained, the hung up their receiving-telephones as a sig- 2 5 attainment of absolute privacy in conversanal of conclusion. Perfect control of the tions between subscribers, and an increase in work is thus attained. Finally, the operators the rapidity, as well as the certainty, of cendepend upon their signals alone for informatral-office operations. Many incidental adtion as to use or non-use of connected lines, vantages will'appear from the detailed dehaving no means to connect their telephones 3o scription, which need not be specifically mendirectly or indirectly thereto after complete tioned. connection is once established. Absolute pri- In order to attain the objects above set out, vacy is thus insured. we have provided certain means, which may i The general scheme of circuits and operabe used in any system, but certain other tion is as follows: All conversation is carried 5 means, relating particularly to central-oifice on over metallic circuits, using the wires connections, which is particularly adapted throughout in series relation, while all sigfor what is known as a trunking or dinals are controlled over the same wires, but videt central system. The first-mentioned on separate circuits, known technically as apparatus we use in exchanges having but phantom-circuits,composed of the line and 40 few subscribers. The second isintended for connecting wires in multiple relation with large systems having many subscribers. The ground or separate wires for return. Both following 'descriptio'n will be based entirely conversation and signaling are thus enabled on the second, for the reason that the first coto be simultaneously effected without mutual operates with it, and in fact forms a compointerference. The conversation or series cir- 5 nent part of it in large exchanges,and the opcuits consist in any case of the line-wires erations are the same whether used in such and connecting-cords, with interposed trunkcooperative relation or separately. The leadlines where the subscribers happen to be coning features then of a large exchange connected to different switchboards. The superstructed in accordance with the invention are visory-signal scheme is based on the princi- 50 as followszAll signals are automatic from the ple of the VVhcatstone bridge, any two line rod .M of the jacks J and J.

wires with their line-signal circuits and the interconnecting office-wires constituting a bridge, whereof two sides contain the linesignals, the other two sides contain the subscribers instruments, the bridge contains the supervisory signal, and an external supplyeireuit, consisting of the earth or a common return,containsa source of signaling-current. In this connection it should be stated that neither line-signals nor coiwersation depend upon the bridge for their eifect.

Our invention is illustrated in the accompanying drawings, in which the same letters and numerals of reference indicate the same parts throughout.

Figure 1 is a diagram showing two subscribers lines with their substation and central-station apparatus,together with one complete central-office outfit for their connection, including trunk and signal circuits. Fig. 2 is a similar diagram showing with more detail the arrangement of a preferred form of subscribers apparatus, and Figs. 3, 4, and 5 are detail views of plugs and their seats.

Referring to the drawings, X and Y are two subscribers stations, provided with receiving-telephones R, transmitters T, and line-wires 1 2 and 3 4, respectively, leading to different switchboards of the same or different central offices. We have shown them so connected because the interconnection of subscribers on the same board or section is accomplished with a portion of the same apparatus and by the same steps.

Bridged across the line-wires at the central office are impedance-coils I I, and connected to the ends of the wires are the springs From the middle point of each impedance-coil a wire 5 is led to one end of windingl of an annunciator or relay magnet L, the otherend of which is connected by wire 6 through the main battery M to ground. The magnet L is provided with another winding Z, laid on oppositely and differentially with respect to the winding Z and connected on one side to the wire 6 and on the other to a contact-spring in the spring-jack. This spring j is so arranged that it is closed upon a contact-anvil j whenever a plug is inserted in the jack, said contact-anvilj being connected through wire 15 and resistance Rh to ground. The magnet L -when energized operates a signal-lamp Z through the medium ofan armature Z adapted to close a local circuit 25, containing the lamp and a suitable source of current. It is not necessary,however, that this signal should be a lamp or that the magnet L should act as a relay, for the armature Z may be so shaped as to serve as a signalitselfin its movement.

The battery M and the resistance Rh are preferably common to all the lines on a section or in the exchange, and it will be observed that the former is never disconnected from the lines, being always ready to send out current over any line that is closed to ground at its other extremity. A constant test is thus maintained, as an accidental ground on either side of a line will manifest itself without delay at the central office.

At each answering-section of the switchboardi. 6., before each answering operator, hereinafter spoken of as the A operator and adapted to cooperate with the springjacks J and J are pairs of plugs P P, each having two contacts connected with the corresponding contacts on its fellow by cord conductors 7 and 8. Interposed in each cordcircuit is an ordinary ringing-key K, adapted when actuated to disconnect plug I from plug P and to connect it to a common circuit containing calling means, such as a generator, in a well-known manner. The plugPof each pair is of ordinary construction and needs no description. It ordinarily rests in a plain seat on the board-table, its cord conductor passing through a suitable orifice in the common way. Each plug P, however, is provided with external metallic rings or conducting portions 0 r on its body, which are electrically connected to its sleeve and tip contacts RR, respectively, and are adapted to make contact with springs 19 and 17, carried in the plug-seat and insulated therefrom, as shown. These springs 19 and p are permanently connected to the two sides 11 12 of the circuit of the operators telephone 0. By this arrangement it is apparent as long as plug P remains in its seat the operators telephone is connected to the cord-circuit 7 8, and hence if the plug P be inserted in aline-jack is thereby connected to line without any further act on the part of the operator.

The apparatus thus far described is sufficient for the connection and disconnection of lines on the same switchboard orsection; but for connecting lines on different sections further means are provided. These consist of three principal appliancesviz., trunk or transfer circuits, operators instruction-circuits therefor, and supervisory signals in the plug-cords.

The trunk-circuits are plain twin-Wire circuits passing between the boards or boardseetions, each terminating at one end in a spring-jack and at the other in a plug. The trunk-jacks J are all placed in front of A operators, while the corresponding trunk-plugs are all placed in front of B operators. The terminal plugs P, as shown, are of the same construction as the plugs P of the cord-circuits. The seats in which they normally rest are provided each with a single contaet-sprin g, however, adapted to make connection with both plugcontacts. Connected to the spring 19 of each trunk-plug seat is a wire leading to asignal S, (shown as an incandescent lamp,) and thence through a resistance Rh to ground. This is the trunk-signal, which indicates to the B operator that its trunk-line has been plugged in on by an A operator at another board.

Vithin each trunk-jack J is provided a cirenitecloser 0, comprising a reciprocating plunger adapted to be moved by a plug when the latter is inserted to its fullest extent in the jack, and springs c 0 adapted to be spread thereby to make contact with anvils c 0 These parts are so adjusted that the plug in its normal position in the jack leaves the circuit-closer untouched, a farther insertion than that required for contact with the jack-springs being necessary to operate the circuit-closer. This operation of the circuit-closer, moreover, is only temporary, the pressure of the springs c and c on the inclined surfaces or cam-faces of the plunger 0 ejecting the same into the position shown in the drawings when pressure is removed from the plug. This action of the springs c and 0 may be assisted, if necessary, by an ordinary helical retracting spring around the stem of the plunger. The springs c c are connected, respectively, to the two sides 13 1 10f the B operators telephone-circuit, while the contact-anvils c c are similarly connected to the circuit 11 12 of the A operator. Thus the depression of the plunger 0 by an entered plug crosses these two circuits for the time. 1 7

Each B operators circuit 13 14 has three terminal pieces of apparatus connected in parallelviz., the telephone set 0, the plunger-switch 0 and a spring-jack J The switch 0 is similar in its construction to the switch 0 and has the same function, that of crossing the B with an A operators circuit. For this purpose each A operators circuit is extended to all the B board-sections, or rather B operators, at each of which it has branch connections to the springs 0 0 which when spread by the plunger 0 make contact with anvils 0 0 The jack J is adapted to receive the terminal trunkplugs on occasion and is technically denominated the busy jack.

Since the ringing-keys which are placed in the cord-circuits 7 8 are exclusively under the control of A operators, it is necessary to provide some means whereby those operators will be enabled to know when to ring and when to stopthat is, when the B operator in any case has inserted the trunk-plug in the jack of the wanted subscriber and when such subscriber has answered. The means provided for this purpose comprise a polarized relay or annunciator magnet N and a signal controlled thereby. The windings of this magnet are included in both sides of the plug-cord 7 8, and it is operativeonly on the passage therethrough in parallel of current of a given directionthat is, the windings circuit containing a signal n.

The magnet is shown The subscribers circuits (shown in Fig. 1) are lacking in detail, being diagraphically represented as they are while the talking set is connected. A fuller diagram is that in Fig. 3, wherein 1 2 are the line-wires, normally closed when the receiver is on the book through the hook-lever H and parallel connected arm h to the circuit of the, ringer Q. WVith the receiver off the hook the line-wires 1 2 are continued by way of hook-lever H and arm 72. to wires 18 19, the two sides of divided induction-coil U, receiver R, and wire g to ground, the receiver being thus inserted in the metallic circuit between the two halves of the induction-coil and having the middle point of its windings connected to ground. The local circuit 20, containing primary winding it of the induction-coil, transmitter T, and a suitable battery, is closed by hook H at the same time that the main circuit is thus completed.

The operation of the system will now be understood. Subscriber X desiring to converse with subscriber Y, removes his receiver R from the hook and places it to his ear. This puts his apparatus in the condition shown in Fig. 1. Current from battery M passes to ground, to the wire g, to the middle point of the receiver-windings, through the two halves of induction-coil U to and through line-wires 1 2 in. parallel to the central ofice, through the two halves of the impedancecoil I to its middle point, through wire 5, the

winding Z of magnet L, and the wire 6 back to the battery. The magnet L is energized by this current and attracts its armature Z displaying the signal Z The A operator, perceiving the signal, inserts the plug P in the jack J, causing the springsj to make contact with the sleeve and tip of the plug, and thus connecting the cord-circuit and her telephone 0 to the line. At the same time the spreading of springsj causes the springj to close on the anvil 3' when a circuit is completed from battery M through wire 6 to winding Z of magnet L, to the spring j and anvil 3' wire 15, and through resistance Rh to ground and back to battery. The current in coil Z neutralizes that in c'oil Zas regards their effect on the core of the magnet L,whose armature l is thereupon retracted and the signal Z retired. Having ascertained from subscriber X that he desires connection with subscriber Y and knowing the latter to be connected to another switchboardsection, the answering operator lifts plug P from its seat, disconnecting her telephone by the act, and places said plug in the jack J on a trunk-line leading to the section whereon Ys terminal jack is located, pressing it into said jack as far as it will go. The first eifect of this insertion of plug P into jack J is to complete a circuit from battery M to ground, to resistance Rh and signal S, to spring contact-rings on plug P and the trunk-circuit 9 10 to the jack J and plug P,

through the cord-circuit 7 8, including the windings in parallel of magnet N, the plug P and jack J, the two halves of the impedancecoil I, wire 5, winding 1, and wire 6 back to the battery. This current lights the lamp S, which is a high-resistance lamp adapted to take the voltage required for the other signals, but does not energize magnet N. The second effect of insertion of plug P into jack J is to cross the circuit 11 12 of A operators set 0 with the circuit 13 14 of B operators set 0 in the circuit-closer O. The A operator then repeats the number required, and theB operator lifts the plug P whose signal is displayed, from its seat and inserts it in the proper jack. As she lifts the plug the signal is of course disconnected and retired. As soon as the plug is inserted in the required jack a new path from battery M is formedviz., from battery to ground, to subscribers station X, to wire 9, and line-wires 1 2 to central jack J, plug P, cord-circuit 7 8, including windings of magnet N, plug P, jack J trunkwires 9 10, plug P jack J, coil I, wire 6, and back to battery. It will be observed that this current flows through magnet N in the opposite direction from that of the signaling-current. The magnet is therefore energized in a proper way to turn its armature n and close the local circuit containing battery 5 and signal n. The battery M as installed in systems of this character always has a low internal resistance, and as the line-wires 1 2 are used in parallel there is always a suflicient flow of current under the conditions thus stated to supply the branch circuit through conductors 7, 8, 9, and 10 no matter what the relative resistances may be. Being thus apprised by the signal a that plug P is in the required jack, the A operator depresses the ringingkey K to send a callingcurrent forward over the trunk and called line. Vhen subscriber Y answers by taking down his telephone from the hook, the signal it disappears. This is because a circuit is then completed to ground at station Y through wire 9 similar in every wayto that at station X, and consequently battery M will send its current over the two lines in parallel, and none will pass through thecordandtrunk line. ThisisatrueWheatstone-bridge effect, and in order that it may be attained in every case without fail the lines are necessarily all brought to the same or approximately the same resistance by the use of artificial resistances inserted, preferably, at the distributing-boards before they reach the switchboards proper. There is a second reason why the line-resistance must be equalized and that is for uniformity in working of the neutralizing-circuit l5. Either each individual branch of wire 15 must be separately balanced with its line, so as not to shunt it when out in, or the lines must all be balanced, and the latter is the most satisfactory scheme. Line-balancing resistances are shown at Rh in Fig. 1. When the two subscribers have finished their communication and have both replaced their receivers on the hooks, current no longer flows through the windings Z of the magnets L and L. These magnets are there'' fore immediately energized by the local windings Z, and both display their signals as clearing-out signals. The hanging up of one of the receivers alone will produce no elfect, except that if the called subscriber only should replace his instrument the magnet N will be energized to apprise the A operator of the fact. As long as either telephone is off the hook current will pass through both magnetwindings 7.

In case the called subscriber does not answer the first ring the A operator, seeing the signal n active, will ring again. If he does not answer at all within a reasonable time, she withdraws plug P from the trunk-jack and restores it to its socket. This immediately places her again in connection with the calling subscriber, so that she can apprise him of the circumstances. At the same time, the

,plug P being still in the jack J, the winding Z is immediately rendered active and the B operator gets a clearing-out signal without knowing, as it is not necessary she should know that the call has not been answered.

In case the connection proceeds up to the point of insertion of plug P into jack J and the B operator discovers that the desired line is already busy, a plug being in that jack, she simply inserts the trunk-plug P in her busy jack J and speaking directly to the calling subscriber says line busy. time she can depress the plunger 0 of switch 0 and the A operator hears the statement, thereupon disconnecting the calling-line.

In this system it will be observed there are no separate clearing out signals and no bridges Whatever of the office-circuits. The impedance-coils I I are bridged across the connected circuits; but they are wound to very high apparent resistance. The subscribers talk through the windings of magnetN; but these windings are of very low resistance and impedance individually, and, moreover, neutralize each other as to currents in the metallic circuit.

Figs. 3, 4:, and 5 show a plug and plug-seat especially adapted for use in the system thus described. Each plug P comprises a tube R, of metal, provided with a shoulder r integrally formed thereon a central spindle vicarrying a tip or head contact R and insulated from the tube R by the conical bushing 0' a heel cup or thimble r, surrounding but insulated from the tube R, and insulated contactrings 1' and 2", carried by and forming part of the tube and the thimble, respectively. In assembling the plug the insulating-sleeve 1" is first slipped on the tube R from the rear end until it rests against the shoulder 0", the ring 0' is screwed in place, the insulatingsleeve 7' is slipped on, the thimble 1" put over it, and the spindle 1 inserted and screwed firmly up in the thimble to bind the whole together. It will be observed that the thimble is centered on the tube over the insulatingsleeve 0', and this centers the spindle 2' with- At the same out further means. A rubber or fiber heelcup r is screwed over the thimble r, which is cut away at one side to expose the end of the tube R, the latter being there provided with a binding-screw r for the cord conductor. The other cord connection with the spindle r is made by means of binding-nuts 1 directly on the end of the spindle. Of a size to receive the plug we provide a tubular seat S, having rubber-hushed side openings 5' and a diametrical cut of some width across its lower end, while at its upper end it has a bead or flange to seat around the edges of the opening that receives it. Seated in the cut at the lower end of the tube S is a perforated block V, of insulating material, with side pieces orwings projecting from opposite sides of the tube and carrying springs 19 20', secured by screws o. These springs 10 19' extend up along and parallel with the tube and at their upper ends are provided with bent contact portions adapted to enter the bushed perforations s and make contact with the rings r and 1"*, respectively, of the plug. The block V is secured 'within the tube S by screws 12 and its central perforation, as shown in Figs. 3 and 5, permits the passage of the conducting-cord from the plug. The rubber bushings prevent the accidental contact of the springs with the tube when the plug is withdrawn, which would short circuit the springs.

We do not wish to be confined to particular details of construction herein set forth or to specific arrangements of the apparatus or circuits, much of which may be changed without departing from the spirit of the invention, but should be understood as including all minor changes within the scope and purview of our description.

Having thus described our invention, what we claim, and desire to secure by Letters Patent, is-

1. In a telephone-exchange system, a subscribers line, a spring-jack therefor, a lineannunciator having differential windings, and a source of current, said source being permanently connected to the line through one winding of the annunciator and adapted to energize it or not according to the condition of the line, in combination with a local circuit containing the secondwinding and means actuated by an operator in making connection with the line to close said local circuitand maintain the same closed as long as the connection continues, substantially as described.

2. In a telephone-exchange system, a subscribers line, bearing aconstantly-impressed electromotive force, means at the subscribers station for determining the flow of current in the line, a doubly-wound annunciator, one of whose windings is connected permanently to line and the other in a normally-open local circuit, also bearing a constantly-impressed electromotive force, the two windings being equal and opposite in their magnetizing effeet, and means actuated by the operator in making connection with the line to close said local circuit, and maintain it closed as long as the connection continues, whereby the subscriber may actuate the annunciator by causing an initial fiow in the line, the operator may restore it by neutralizing the effects of said flow, and the subscriber may again ac-' tuate it by causing the flow in the line to cease, substantially as described.

3. In a telephone-exchange system, subscribers lines, and combined calling and clearing-out annunciators therefor, each consisting of a doubly and differentially wound magnet and its armature, a constantly-active source of current, means under the control of a subscriber to direct current from said source through one winding of his annunciator, means under the control of an answering operator to direct current through the other winding, and further means under the control of the subscriber to cut off the current from one winding, leaving the other active, substantially as described.

4. In a telephone-exchange system a subscribers line, and an an nunciator and springjack therefor at the central office, the annunciator being provided with double windings laid on differentially, one winding connected to the line and the other in a local circuit, a constantly-active source of current, means at the subscribers station, actuated in switching the telephone for use, to open and close circuit through said source and through the line-winding of the annunciator, and means connected with the spring-jack, and actuated by an operator in making connection there- -with, to close the local circuit through a source of current while such connection continues, substantially as described.

5. In atelephone-exchange system two subscribers lines, each permanently connected through a line-annunciator to a source of current, and a common return-conductor sepa rate from the lines, extending from the source of current to the subscribers stations, central-office-connecting means bridged across the lines and forming a metallic-circuit path for conversation-currents, a supervisory signal included directly in the said connecting means, and switches at the subscribers stations for determining the flow of current in the respective lines, substantially as described.

6. In a telephone-exchange system, a subscribers line permanently connected at one switchboardsection through aline-annunciator winding to a source of current, and provided with a terminal jack; a trunk-line extending to another switchboard-section and connected to a signal at its farther end, and means to connect said line-jack to the trunkline, the arrangement being such that when the line is so connected current will flow from the line through the trunk-line and cause the exhibition of the signal, substantially as described.

7. In a telephone-exchange system a subscribers line having a terminal jack, a pair of plugs and a metallic-circuit-connecting cord consisting of two conductors, the second plug of the pair having a tip and a sleeve connected respectively to the two conductors of the cord, a pair of rings carried on the body of the plug and insulated from each other, one joined to the tip-contact, and the other to the sleeve, together with a plug-seat carrying a block of insulation with a pair of springs thereon connected respectively to the opposite sides of an operators circuit, the springs being of different lengths, so as to make contact respectively with the two rings on the plug when the latter is in the seat, whereby an operators circuit is directly connected with a subscribers circuit by the mere act of inserting an answering-plug, substantially as described.

8. In a switch-plug fortelephone systems the tube constituting a sleeve-contact,the central spindle terminating in a tip-contact, the cone of insulation underlying the tip and fitting within the end of the tube, the insulating-jacket on the tube, and the surrounding cup centered thereon, cut away to expose the end of the tube, and receiving and centering the end of the spindle, substantially as described.

9. In switching apparatus the plug consisting of a tubular body, an insulated headed spindle therein, a rear cup surrounding the body but insulated therefrom and receiving the end of the spindle, and exposed conducting portions on said body and cup; in combination with a recessed seat, and insulated contact-springs therein adapted to rest on said conducting portions of the plug when it is in the seat, substantially as'described.

10. In switching apparatus the plug-seat consisting of a tubular shell, a block of insulation secured to the lower end thereof and perforated to permit the passage of the plugcord, hushed orifices in the sides of the shell at different distances from the block, and contact-sprin gs secured to the block and ex tending through the orifices within the shell, substantially as described.

11. In a telephone-exchange system the combination of subscribers lines connected to a common source of current at the central ofiice, and arranged to permit the current to flow over each pair of Wires in parallel when the circuit is closed at the subscribers station, a central-office circuit for connecting one pair with another for conversation, and a supervisory-signal magnet included therein having its windings oppositely connected in the two sides thereof, said connecting-circuit forming a bridge of the connected lines, whereby it only one subscriber has his circuit closed the operator will be apprised thereof, substantially as described.

12. In a telephone-exchange system, subscribers lines connected through a common source of current at a central office to ground or a common return, and provided with switches at the subscribers stations to complete the ground or common-return circuit, connecting cord-circuits and trunk-1ines,each trunk-line provided with a terminal signal device, connected on one side to said commonreturn conductor and each cord-circuit containing a polarized supervisory-signal magnet, whereby, first, when a line is connected through a cord-circuit to a trunk-line the signal of the latter will be displayed but the polarized supervisory-signal magnet will not respond, second, when the trunk-line is connected to asecond line current will flow back from said line in a reversed direction through the cord-circuit to energize the supervisorysignal magnet, and third, when the called subscriber connects his line to ground or common return current will cease to flow in the cord-circuit and the supervisory-signal magnet will become inert, substantially as described.

13. In a telephone exchange system, a \Vheatstone bridge of which each of two connected subscribers lines, from the substation to the spring-jack, forms a side; each subscribers-line connection from the spring-jack to a central battery, through a line drop-annunciator, forms another side; said batter Y and a common-return conductor, or ground, connecting it to all the subscribers stations and to switch-contacts therein; the oflice-connective circuit or operators cord-circuits, forming the bridge-conductor, from jack to jack of the connected lines; asupervisory signal included in said offiee-connective or cord circuit; and means whereby either subscriber may change the switch-contacts at his station to thereby change the condition of his side of the IVhcatstone bridge, whereby current from the central battery will flow through the bridge conductors and'energize the supervisory signal, substantially as described.

WILLIAM D. GHARKY. EDWARD E. CLEMENT.

Witnesses:

ROBERT OSBORNE, .Tr., EMERSON CONRAD. 

